The History of Cosmology

Introduction to The History of Cosmology

Human efforts to make sense of the universe undoubtedly began long before the dawn of civilization, but the speculations of the first stargazers can never be known. We do know that more than 2,000 years ago among the ancient Greeks, at least one person—the philosopher Aristarchus—believed that the Earth and other planets revolved around the sun.

Aristarchus and another Greek who came before him, Democritus, also pondered the stars, theorizing that they were other suns. Democritus was one of the boldest thinkers in ancient Greece. He proposed that everything is composed of atoms and that the hazy band of light across the sky formed by our own Milky Way galaxy was made up of stars too far away to be seen as individual pinpoints of light.

The ideas of Aristarchus and Democritus were revolutionary for their day. If their views had prevailed, the modern age of cosmology might have begun much sooner.

Aristotle and Ptolemy

But most Greek scientists and philosophers, including the influential Aristotle and Plato, held other beliefs. They thought the Earth was at the center of everything, with the sun, stars, and planets circling about it affixed to transparent celestial spheres. As these vast spheres rotated, the heavenly bodies attached to them traced out circular orbits around the Earth.

According to Aristotle, who summed up the astronomical thinking of his day, the outermost sphere contained all the stars. But neither he nor any of the other ancient Greeks had any idea how huge the universe is. They estimated that the stars were a few thousand, or perhaps a few million, miles away. The nature of the stars was something they did not try to explain.

In the A.D. 100's, the basic scheme outlined by Aristotle was taken even further by the most famous of the early astronomers, Ptolemy, a Greek who lived in Alexandria, Egypt. Because the invisible celestial spheres helped explain the movement of heavenly bodies, Ptolemy retained them in his view of the universe, though he rejected the idea that they were actual, solid objects. But despite that simplification, Ptolemy's system added new complications to account for the motions of the planets across the sky.

Although it was cumbersome, the Ptolemaic system accounted quite well for the observed motions of the heavenly bodies, so it went unchallenged for nearly 1,500 years. But by the late Middle Ages, it had become evident to careful observers of the skies that Ptolemy's universe did not provide a truly accurate description of celestial phenomena. The time was ripe for a new view of the cosmos.

The Influence of Copernicus

That new cosmic view arrived in 1543 with a book titled On the Revolutions of the Heavenly Spheres, by the Polish astronomer Nicolaus Copernicus. Like Aristarchus, whom he may have read about in the writings of the ancient Greek author Plutarch, Copernicus put the sun at the center of the solar system. Copernicus was motivated largely by a philosophical conviction that the structure of the universe must be simple and elegant. The Ptolemaic universe was anything but. Today, we date the modern era of astronomy and cosmology to the “Copernican Revolution” fostered by Copernicus' groundbreaking book.

But Copernicus' theory failed to gain immediate acceptance because many people were not ready to give up the idea that Earth was the center of the universe. Religious authorities, in particular, were committed to an Earth centered cosmos. Realizing that he risked being denounced for contradicting the official position of the church, Copernicus had delayed publishing his book until he was on his deathbed.

Copernicus' theory, moreover, could not account precisely for the movements of the planets. Like the Greeks, Copernicus had assumed planetary movements to be circular—cross sections of great invisible spheres. The circle and sphere were perfect geometrical forms, and the heavens simply had to be the embodiment of perfection. But nature pays no attention to human ideas about how it should behave. The planets do not move in circular orbits, as the German mathematician and astronomer Johannes Kepler discovered in the 1600's.

Kepler and Galileo

Kepler was an avid supporter of the Copernican system, and he was determined to discover why it could not account for what astronomers observed. In 1600, Kepler became an associate of the Danish astronomer Tycho Brahe, who had spent years making detailed records of planetary motions. Brahe died in 1601, but Kepler continued to study Brahe's data, trying to make them fit the idea of circular orbits. Finally, Kepler realized that circular orbits were simply not correct, and it was then that he found the answer. The planets, he proclaimed in 1609, move in elliptical (oval) paths.

A contemporary of Kepler's, the Italian astronomer and physicist Galileo Galilei, advanced celestial observations with an important new tool: the telescope. Using a telescope to view the heavens, Galileo saw that the moon is covered with mountains and craters and that Venus, like the moon, goes through regular phases, changing in appearance from crescent shaped to a round disk.

The stars were a particular surprise to Galileo. They not only apparently numbered in the millions but also varied tremendously in their range of brightness. Most stars were much too faint to be seen by the unaided eye. This finding indicated that some stars were considerably more distant than others, rather than all being equally far away, as most people had assumed since at least the time of Aristotle. The perceived universe was growing ever larger.

Galileo's observations greatly strengthened the Copernican hypothesis. For example, the fact that Venus goes through phases like those of the moon indicated that Venus circles the sun. Although absolute proof that the Earth was not the center of the universe was still lacking, Galileo became an outspoken critic of all who expressed doubts about the Copernican system. As a result, angry church authorities in Rome brought Galileo to trial in 1633 and forced him to renounce his belief in Copernicanism. Their victory, however, was nearly the last gasp in efforts to preserve the notion of an Earth centered cosmos.

Newton and Herschel

In 1642, the year of Galileo's death, one of the greatest scientists in history, Isaac Newton, was born in England. Newton's interests and accomplishments were far ranging, but it was his studies of gravity that were of most importance to the advance of astronomy. Newton discovered that the same gravitational force that causes an apple to drop to the ground from an apple tree keeps the planets in orbit about the sun and holds the universe together.

Newton used his theory of gravity to calculate how the planets should move around the sun. The answers to his equations revealed that the planets should move in elliptical orbits, as Kepler had found from his analysis of Brahe's observational data.

Another scientist in England, William Herschel, became the preeminent telescope builder and astronomer of the late 1700's. Using reflecting telescopes (telescopes that gather and focus light with mirrors rather than lenses) up to 122 centimeters (48 inches) in diameter, Herschel observed the panorama of the heavens in even greater detail.

Much of Herschel's time was spent studying glowing patches of light called nebulae. Astronomers earlier in the 1700's had begun finding these unusual objects, many of which they noted were disk shaped. They assumed that the nebulae were part of the Milky Way, because the Milky Way was then thought to be the entire universe.

Other observations had suggested that the Milky Way itself was shaped like a disk. In a flash of insight, the German philosopher Immanuel Kant had proposed in 1755 that the disk shaped nebulae were other galaxies like the Milky Way but far removed from it. “Island universes” he called them.

Seeking to resolve this question, Herschel scanned the heavens for nebulae and discovered about 2,000 new ones. He determined that many were huge clouds of gas or clusters of stars in the Milky Way. The disklike nebulae, however, appeared only as hazy smudges to his gaze.

Observations by later astronomers revealed that many of the disk shaped nebulae had spiral shapes, but still no stars could be seen in them. So whether the nebulae lay within the Milky Way or were separate galaxies remained an open question for many years.

Einstein

While astronomers surveyed the heavens, physicists were learning how the universe works. The greatest of these researchers, whose name would rank with Newton's, was Albert Einstein, who was born in 1879 in Germany.

In 1915, Einstein proposed a general theory of relativity, a new explanation for gravity. The theory described gravity in geometric terms, as a curvature of space. In this view, a planet or other large body warps the space around it, much as a bowling ball placed on a mattress would form a depression in it. The warping of space causes smaller objects to move in curved paths toward a massive object, just as marbles would roll toward the bowling ball because of the depression in the mattress.

Einstein also pondered the nature of the universe as a whole. The theory of relativity predicted that the universe could not exist in an unchanging condition, but rather had to be either expanding or contracting. But at that time, all the evidence pointed to a static universe. Rather than insisting that his theory was accurate, Einstein altered his equations by adding a factor he called the “cosmological constant,” which counteracted gravity to produce a changeless universe.

A Universe of Galaxies

Einstein would soon have reason to discard the constant and call it the biggest error of his career. Discoveries made in the 1920's by the American astronomer Edwin Hubble of the Mount Wilson Observatory in California showed that Einstein's original conception of the universe had been correct after all. In addition, Hubble finally settled the question of whether the spiral nebulae are other galaxies.

Hubble studied stars called Cepheid variables, which periodically become brighter, dimmer, and then brighter again. The time between peaks of brightness can be a few days or several months.

In 1912, Henrietta Swan Leavitt, an astronomer at the Harvard University observatory in Cambridge, Mass., had made a key finding about the Cepheids. She discovered that the longer the time period between a Cepheid's peaks of brightness, the greater the star's true brightness. Cepheids with the same period—the same length of time from peak brightness to peak brightness—always have the same maximum true brightness. (A star's true brightness, as opposed to its relative brightness as observed from Earth, is the actual amount of light it gives off. A faraway star might have a higher true brightness than a nearby star, but because of its distance, it has a lower relative brightness.)

Leavitt's finding meant that Cepheids could be used as cosmic measuring posts. Because the brightness of an object varies with its distance in a known way, it would be a simple matter to calculate the distance of a remote Cepheid. This could be done by comparing the star's relative brightness to a nearby Cepheid of the same period whose distance had already been calculated by other means.

In 1925, Hubble reported that he had found Cepheid variables in spiral shaped nebulae and had calculated that the stars were much too far away to be within the Milky Way. Thus, the spiral nebulae were galaxies in their own right. Another class of nebulae, the ellipticals, with an oval shape but no spiral arms, were also soon recognized as being galaxies. The universe, astronomers now realized, consisted of billions of galaxies, each containing billions of stars.

Discovering the Expanding Universe

Beginning about 10 years earlier, an astronomer at the Lowell Observatory in Flagstaff, Ariz., Vesto M. Slipher, had begun obtaining data that would prove useful to Hubble in his next great discovery. Studying the light from spiral “nebulae”—their true nature as galaxies had not yet been determined by Hubble—Slipher found that in almost every case the light was shifted toward the red end of the spectrum.

This phenomenon, called the red shift, had been used by astronomers since the 1800's to measure the speeds of stars orbiting the center of the Milky Way. The red shift occurs because light waves emitted by a rapidly receding source become stretched out as they move away from the source. The stretching effect increases the wavelength of the light. Longer wavelengths fall toward the red end of the spectrum. In contrast, light waves from a source moving rapidly toward an observer are compressed and shifted toward the short wavelength blue end of the spectrum.

Hubble was intrigued with Slipher's finding that the light from the spiral nebulae (spiral galaxies) was red shifted. This could only mean, he said in a 1929 research paper, that all the other galaxies are rushing away from us at high speed. The universe, as Einstein's theory had predicted, was expanding.

The Emergence of the Big Bang Theory

But if the universe truly was expanding, why was it doing so? By the late 1940's, two theories had emerged to account for the expansion of the universe.

Three British astrophysicists—Hermann Bondi, Thomas Gold, and Fred Hoyle—developed the steady state theory. According to this theory, the universe has always existed and has always looked essentially the same, except that galaxies constantly move away from each other. In the growing empty regions between them, new stars and galaxies are formed from matter that somehow pops into existence from the vacuum of space.

A different scenario was presented by the big bang theory, whose most outspoken proponent was the Russian-American physicist George Gamow. The big bang theory held that the universe began in a huge explosion of matter and energy at some moment in the far distant past. Matter created in this “primeval fireball” later came together to form the stars and galaxies.

In 1948, Gamow predicted that even though the universe had greatly expanded and cooled since the big bang, the explosion's leftover energy would still exist as very faint microwaves (high frequency radio waves) throughout space. Detecting this cosmic background radiation would lend strong support to the big bang theory.

Gamow's prediction was not confirmed until the 1960's. In 1965, two researchers at Bell Telephone Laboratories in Holmdel, N.J.—Arno A. Penzias and Robert W. Wilson—were working with a large horn shaped antenna designed for satellite communications. They detected a persistent hiss from wherever in the sky they pointed the antenna. That hiss was the sound of the cosmic background radiation.

With Penzias and Wilson's discovery, the big bang theory became widely accepted as the correct explanation for the origin of the universe. Although the theory has been refined, with details added by physicists studying matter on the smallest levels, the “big picture” of our cosmos being born in a moment of fiery energy appears to be correct. We live in a universe that has been here for a limited amount of time and that is evolving toward a future we have yet to determine.